A car that doesn't need gas, or at least not much, is getting slightly more realistic all the time.

A few small companies will start to offer services and products for converting hybrid cars like the Toyota Prius that currently get around 50 miles per gallon into plug-in hybrids that rely more heavily on electrical power and can get about 100 miles per gallon.

"I get about 99 miles to the gallon," said Felix Kramer, founder of The California Cars Initiative (CalCars), who owns the eighth Prius converted into a plug-in hybrid. "When gasoline costs $3 a gallon, driving most gasoline cars costs 8 to 20 cents a mile. With a plug-in hybrid, your local travel and commuting can go down to 2 to 4 cents a mile."

In general, plug-in hybrids have much larger battery packs than standard hybrids--in prototypes, the extra batteries fill up the space where spare tires now reside--and much smaller gas motors. The batteries can be recharged by plugging the car into any wall socket.

Under 34 miles per hour, the electric motor effectively powers the car on its own, said Kramer. Over that--and during bursts of acceleration--the gas motor begins to help incrementally. The gas motor also takes over when the battery conks.

"Sixty-five percent of drivers will not use gas on a daily basis. The only time you ever use gasoline is when you go on vacation or go skiing," said Andrew Frank, a professor of mechanical engineering at the University of California at Davis who has made plug-in hybrids out of stock Mercury Sables and a Chevy Suburban. The Suburban has been tested on General Motors' off-road track.

"It would do the same thing as a conventional Suburban, including towing a trailer," he added.

It all comes down to cost
But conversion won't be cheap--at least initially. California's EDrive Systems will charge around $10,000 to $12,000 to install the extra lithium batteries needed to turn a standard Prius into a plug-in hybrid when its service begins later this summer.

At that price, and with gas at $3 a gallon, it would take around 160,000 to 200,000 miles of driving to break even. As a result, conversion services today are really being sold more as a luxury option or status symbol.

But some groups are looking to the do-it-yourself crowd for a cheaper solution. Canada's Hymotion, which already converts fleets of hybrids for corporate customers, will charge about $9,500 for a kit aimed at consumers that it will start shipping in October. And Hymotion can convert more than just the Prius.

CalCars is working with independent inventors to bring the price of a DIY kit based around an open blueprint to about $3,000.

"Our goal for the build kit is this summer, but making this happen will be a volunteer project--as are most open-source efforts--so I'm not in a position to promise," Kramer said.

Mass manufacturing, though, could lower the prices dramatically over time. Frank estimates that a plug-in hybrid with a 60-mile range (meaning the car can run on electricity alone for up to 60 miles) might cost only $6,000 to $7,000 more to mass manufacture than a conventional car in a few years. A standard hybrid currently goes for about $3,000 more than gasoline-driven cars.

To get to that point, however, battery technology, which tends to progress slowly, will need to improve. Auto manufacturers will have to improve the transmissions and other components that go into a hybrid.

The high cost is one of the primary reasons that major auto manufacturers have been lukewarm to the concept of plug-in cars, engineers at large auto manufacturers have said. Finding ways to stash the battery without compromising passenger or cargo room is another.

Nonetheless, some automakers have shown interest. DaimlerChrysler will produce 40 plug-in versions of its Sprinter minivan for testing the concept. No commitment has been made to turn it into a product.

Pollution-free
Over several years, the cars also can pave the way toward nearly pollution-free cars, said Frank. Because gasoline consumption is modest, it will likely be possible to build plug-in hybrids that burn ethanol rather than gas.

For electricity, the cars could harvest solar power from solar panels installed in garages or houses. Although electric motors don't pollute, electricity gets generated in coal-burning plants, one of the largest sources of greenhouse gases.

"We can't switch from where we are today overnight. It will take 20 years or more to take the PHEV (plug-in hybrid electric vehicle) to get into our society," Frank said. Nonetheless, "we can greatly reduce the amount of liquid fuel we use for transportation," he said.